In electrophotography, a latent image is created on the surface of an insulating, photoconducting material by selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and unexposed to light. The visible image is developed by electrostatic toners containing pigment components dispersed in an insulating binder. Two types of developer materials are typically employed in the electrostatic imaging process. The first type of developer material is known as a dry developer material and comprises toner particles, or carrier granules having toner particles adhering tribo-electrically to the carrier granule. The second type of developer material is in the form of a liquid developer, comprising a liquid carrier having toner particles dispersed within the liquid carrier. The toners are selectively attracted to the photoconductor surface areas either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor surface, development electrode and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. For laser printers, the preferred embodiment is that the photoconductor and toner have the same type, but different levels of charge.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and passed close to the photoconductor surface, pulling the toner from the photoconductor surface onto the paper or intermediate medium still in the pattern of the image developed from the photoconductor surface. Thermal energy may also be used to assist transfer of the image to paper or intermediate transfer medium. For the case where no thermal transfer is used, a set of fuser rollers melts and fixes the toner in the paper subsequent to direct transfer or indirect transfer when using an intermediate transfer medium, producing the printed image.
There is a demand in the laser printer industry for multi-colored images. In color electrophotography, four color planes have to be constructed and accumulated either on the photoconductor, on an intermediate roller/drum or on the media. In liquid color, electrophotography storage on an intermediate has been the preferred approach. In dry powder, color electrophotography all techniques exist in the art.
With liquid toners, it has been discovered, the basic printing colors of yellow, magenta, cyan and black, may be applied sequentially to a photoconductor surface, and from there to a sheet of paper or intermediate medium to produce a multi-colored image. Liquid carriers used with liquid toners have characteristically exhibited high volatility. Such a highly volatile carrier liquid is easily evaporated by heat, pressure and/or air flow, thereby assuring a dry media sheet upon completion of the printing process. Such volatile carriers are flammable and consequently are impacted by today's strict regulations regarding the handling of flammable materials.
In dry powder, color electrophotography there are electrophotographic engines which store the four color planes on the photoconductor, on an intermediate or on paper. All of these devices use electrostatic transfer of the four color planes to the media followed by a fusing step. A key disadvantage of these systems is the requirement for independent fusing stations and/or complex media separation techniques. Another major disadvantage is the toner charge alteration which occurs during electrostatic transfer over the environmental range due to air breakdown and concomitant toner resolution loss that occurs with a second electrostatic transfer.
A single electrostatic transfer system represents an advantage by providing economy in terms of printing supplies, improved image resolution and, in the case of liquid toner, eliminating environmental and health concerns from disposal of excess liquid carrier medium.